Positive Homotropic Allosteric Receptors for Neutral Guests: Annulated Tetrathiafulvalene–Calix[4]pyrroles as Colorimetric Chemosensors for Nitroaromatic Explosives

Abstract

The study of positive homotropic allosterism in supramolecular receptors is important for elucidating design strategies that can lead to increased sensitivity in various molecular recognition applications. In this work, the cooperative relationship between tetrathiafulvalene (TTF)-calix[4]pyrroles and several nitroaromatic guests is examined. The design and synthesis of new annulated TTF-calix[4]pyrrole receptors with the goal of rigidifying the system to accommodate better nitroaromatic guests is outlined. These new derivatives, which display significant improvement in terms of binding constants, also display a positive homotropic allosteric relationship, as borne out from the sigmoidal nature of the binding isotherms and analysis by using the Hill equation, Adair equation, and Scatchard plots. The host-guest complexes themselves have been characterized by single-crystal X-ray diffraction analyses and studied by means of UV-spectroscopic titrations. Investigations into the electronic nature of the receptors were made by using cyclic voltammetry; this revealed that the binding efficiency was not strictly related to the redox potential of the receptor. On the other hand, this work serves to illustrate how cooperative effects may be used to enhance the recognition ability of TTF-calix[4]pyrrole receptors. It has led to new allosteric systems that function as rudimentary colorimetric chemosensors for common nitroaromatic-based explosives, and which are effective even in the presence of potentially interfering anions.

A colorimetric receptor combined with a microcantilever sensor for explosive vapor detection

“…This receptor has previously been demonstrated to bind TNB in organic solution. 5 However, although superior to earlier TTF-functionalized calixpyrrole receptors, is capable of detecting TNB in solution with a limit of detection of approximately 0.3 ppm in halogenated solvents. We have now incorporated this receptor on a cantilever sensor in the hopes of improving the sensitivity and creating a device capable detecting TNB vapors.…”

“…[2][3][4][5] Their ease of use and their ability to function in the absence of any supporting instrumentation could make them applicable in certain-situations, where a qualitative yes/no indication involving the presence or absence of a targeted explosive is required. However, in spite of their potential advantages, the colorimetric sensors for nitroaromatic explosives reported to date suffer from disadvantages.…”

“…To achieve this objective, we developed a polymer microcantilever with an integrated deflection sensing element. We coated the cantilever with a layer of TTF-C4P, a colorimetric sensing material, which represents a generalized class of receptors 5 that is known to provide a colorimetric response to nitroaromatic explosive TNB. The device presented in this paper was found capable of detecting TNB in vapor phase via process that involves a change in the surface stress of the cantilever.…”

Functionalization reactions of calixarenes

“…The large number of publications dealing with calixarenes is a reflection of their various applications. Calixarenes are receptors for neutral and charged species, [18][19][20] they form complexes with metal ions, [21][22][23] and are promising for environment protection due to their ability to extract Related to calixarenes also are the calixcrowns, 53,54 which are calixarenes bearing crown ether units; calixquinones, 55,56 containing one or two quinone units instead of phenol moieties of calixarenes; calixpyrroles, [57][58][59] built from pyrrole units, and calixphyrins, 60,61 formed by the reduction of porphyrins. Pillarenes, [62][63][64] in which benzene rings are linked by para, and not by meta positions as in calixarenes should also be included.…”

Synthesis and recognition properties of higher order tetrathiafulvalene (TTF) calix[n]pyrroles (n = 4–6)

“…9 In this prior work (wherein anion affinities were not considered per se), it was concluded that 1 : 2 complexes, such as 2$(TNP) 2 and 2$(TNB) 2 , are formed, both in solution and in the solid state. This conclusion was reached on the basis of both continuous variation plots showing a maximum at 0.33 and X-ray crystallographic analyses.…”